Not "WotC" canon, but published in my PDF book, the Enchiridion of Mystic Music and AFAIK the only hard-and-fast rule out there (haven't seen another). Side notes: Feats are available to boost the maximum range, and the "keeps the ability to discern words and notes at 120' rule" is actually fairly accurate IRL (based on 4 years of experience in a marching band). This is playing/yelling as loud as you can so people can hear you at the greatest possible distance.
Hearing Ranges for Mystic Music Effects
Under normal conditions, hearing range is a 120 foot spread (i.e., it can turn corners and so forth). The spread can make its way through solid items (such as doors and walls) at a cost of 10 “feet” per point of Hardness possessed by the item. Treat areas of silence as though they were impenetrable walls of force for determining the reach of the spread. This is the range at which a single musician can be heard sufficiently well to affect a target with a mystic music ability. Each additional musician beyond the first increases the range of the spread by 40 feet (while not strictly correct physically, it is the “closest fit” for groups of fewer than 8 musicians). If the musician is playing in conditions that would hinder Listen checks, this range is adjusted by –10 feet for each –1 penalty to a Listen check. This means that a musician playing in high winds, for example, where a –4 penalty applies to Listen checks, would have hearing range reduced by 40 feet to a total of 80 feet. A musician may always choose to play more softly and decrease the hearing range.
Note that it is possible to hear the music that a musician is playing outside of this range; it is simply too indistinct to allow the effects of the mystic music to be effective. Anyone attempting to perceive that the musician is playing who is outside of the “hearing range” must succeed at a Listen check (DC 15 + 1 for every 10 feet between the subject and the hearing range). For example, in normal conditions, a character 200 feet away from a musician who attempts to Listen to see if the musician is playing must succeed at a Listen check against DC 23 (he is 80 feet outside of the hearing range of 120 feet, so the DC is 15+8).
--The Sigil